1. Field of the Invention
The present invention generally relates to calibration of a robot. More particularly, the invention relates to automatic calibration of a robot in a processing system.
2. Background of the Related Art
The use of robots is convenient wherever and whenever the operations to be executed are high-precision, repetitive, tedious, stressful or otherwise hazardous to humans. Industries such as automobile manufacturing, production and assembly of integrated circuits, material handling and many others have employed industrial robots to improve efficiency and increase productivity.
Many robots share the same basic design incorporating a base with one or more movable arms attached to the base. The arm typically includes a number of rigid bodies or segments connected by joints with a tool or an end effector usually mounted on the last segment. While the base is stationary, the joints are free to move in “revolute” or “prismatic” fashion. Revolute joints are hinged to provide motion along an arc while prismatic joints are telescopic or extendible to provide linear motion. The end effector is typically provided with a mounted gripper or some other device useful for manipulating objects. For example, end effectors for handling semiconductor wafers or other objects with flat and smooth surfaces are equipped with a robot blade, a vacuum chuck or other means for holding and supporting the wafer or object to be transported. After securing the object with the end effector, the robot can move or change the position of the object. When the robot is automated or computer-programmed, it can perform these tasks repeatedly. This is advantageous, but requires a few preparatory steps. In particular, the robot has first to be “shown” or “taught” what to do. The process of showing or teaching the robot is generally called calibration.
In the manufacture of integrated circuits, semiconductor substrates are loaded into various reaction and other chambers using automated equipment for processing. Equipment has been designed including a robot that can transfer a semiconductor substrate, such as a silicon wafer, from a cassette through a central transfer chamber and into one or more processing chambers connected to the transfer chamber. The robot is disposed within the transfer chamber and provides access to the process chambers connected to the transfer chamber. It is desirable to position the substrate at an optimum location within the processing chamber to maximize the effectiveness of the processing onto the precise desired surface area of the substrate to be processed.
According to one simple teaching method, the robot is taken through its operational path, from the point where the end effector grasps the object or workpiece to the point where it releases it. At certain points in its operation path, the robot is stopped and the joint values are recorded. Many conventional robots have internal systems that automatically report joint values. These joint values can be fed to the robot memory for later reference. When the time comes for the robot to do its job, it “remembers” the joint values that correspond to the desired position and performs its task accordingly.
Currently, robots used in substrate processing systems are typically calibrated using a manual calibration method. The robot is instructed to move according to inputs from a human operator into the robot movement control system. The human operator visually observes the position of the robot within the processing system and plans the path of the robot to avoid obstructions in the processing system. The human operator then manually inputs signals that instruct the robot to move from an initial position to a destination position. To determine that the robot has reached its destination position, an alignment pin is inserted through a calibration hole on the end effector to a corresponding calibration hole at the destination position. The destination position is reached when the alignment pin aligns the matching calibration holes. The movement of the robot from the initial position to the destination position is recorded so that it can be repeated when the same action is required.
However, there are several disadvantages to the manual calibration method. First, manual calibration relies on the ability of a human operator to properly align elements and introduces the possibility of human error. Also, the manual calibration method also requires a considerable amount of time when many locations are to be trained. Furthermore, the speed and accuracy of the manual calibration method mainly depends on the ability of the human operator to perform the calibration. Thus, the manual calibration method has not been satisfactory for the high-precision robot training, particularly for robots used for substrate processing systems.
Therefore, there is a need for an apparatus and a method for automatic calibration of robots, particularly robots used in substrate processing systems. It is desirable for the automatic calibration apparatus and method to achieve fast and accurate calibration while reducing dependence on the ability of human operators. It is further desirable for the automatic calibration apparatus and method to provide reliable calibration and be adaptable for use with a variety of robots.